1. Field of the Invention
The present invention relates to an optical probe which in use is mounted on the movable arm of a coordinate positioning machine to enable the machine to determine the position of a surface. More particularly, the invention relates to a non-contact probe which, upon detection of a surface, emits a trigger signal which is sent to the machine control to cause it to record the position of the movable arm at the instant of trigger, thereby to determine the position of the surface.
2. Description of Related Art
It is known to provide a non-contact trigger probe in the form of a video camera which projects an image of a workpiece to be measured onto a charge-coupled device (CCD) array. The CCD array consists of a large number of pixels each of which outputs an electrical signal corresponding to the intensity of light incident thereon. The known video trigger probe emits a trigger signal indicating detection of the edge of a surface when the value of the output signal of a predesignated pixel passes above or below a threshold level. The problem with such a known probe is that the image processing circuitry in the probe sequentially scans each pixel in the CCD array; the time period for a single scan of the entire array being of the order of 20ms. There is therefore a possible time band of 20ms during which the output of the predesignated pixel may have passed the threshold level. The accuracy of a measurement made with this known probe is therefore limited to the speed at which the image processing circuitry may scan the entire CCD array.